Documentation consistency with the Python interface

[dorchard]

This might be a question of updating the docs, but I notice that the description of the BMI interface for get_value in Python says that
it should return None: https://bmi.csdms.io/en/stable/bmi.getter_setter.html#get-value

But the Python abstract class description has it returning an NDArray:

bmi-python/src/bmipy/bmi.py

Line 344 in aca0289

def get_value(self, name: str, dest: NDArray[Any]) -> NDArray[Any]:

In contrast, set_value returns None

bmi-python/src/bmipy/bmi.py

Line 408 in aca0289

def set_value(self, name: str, src: NDArray[Any]) -> None: